Water stored for years in water butts

I have numerous water buts dotted around the plot along with 2 galvanised troughs that collect water from 2 old pig shelters, a lot of them have green algae on them for much of the year. The water below seems fine as I use it throughout the year. I have just erected a new frame to take the large container I use, because it has a large surface area it tends to green up I think because it gets a lot of sunshine. I have taped some polythene to the poly tunnel and draped in the container to collect water from the roof and connected it to an irrigation system, all I need now is some rain.